The present invention relates to a gas concentration measuring apparatus for measuring a concentration of a specified gas component contained in a gas.
Gas sensors are utilized for suitably controlling automobile engines in recent years. The gas sensors are configured to measure a concentration of a specified gas component, such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbon (HC), oxygen (O2), or the like, which is contained in exhaust emissions exhausted from the engines.
As one of the gas sensors, a gas sensor designed to output a sensor current according to the concentration of the specified gas component is generally used.
A gas concentration measuring apparatus including such a gas sensor is configured to measure the amount of the sensor current to measure the concentration of the specified gas component contained in a gas exhausted from an engine, an example of which has been disclosed in U.S. Patent Publication 6,547,955 (Japanese Patent Publication 2000-171435).
In this type of conventional gas concentration measuring apparatus including the gas sensor configured to output a sensor current in accordance with a concentration of a specified gas component, the sensor current outputted from the gas sensor is so weak as to be subject to electrical noises or the like.
For instance, the sensor current may be outputted as weak current of about a few nanoamperes (nA) from the gas sensor that measures the concentration of NOx as the specified gas component, requiring high measurement accuracy in 1 nanoampere (nA).
On the other hand, as materials of circuit substrates where electrical circuits are formed, an insulating material, such as glass epoxy resin, can be utilized. The circuit substrate made of an insulating material may cause weak leakage current to flow between copper patterns formed thereon that electrically connect electrical components with each other. Especially, under adverse environment of, for example, high temperature, high humidity or the like, the surface resistance of circuit substrate decreases, thereby excessively increasing the leakage current.
For example, when a circuit substance made of glass epoxy resin as the insulating material is used under high temperature and high humidity, the substrate resistance between the copper patterns adjacent to each other at intervals of about 0.5 mm may decrease up to about 1010 ohms (Ω). In this case, there is the possibility that leakage current of about 1 nA (nanoampere) occurs due to a potential difference of about a few volts applied between the adjacent copper patterns.
As described above, in the conventional gas concentration measuring apparatus, the leakage current occurring in the circuit substrate used for measuring the sensor current outputted from the gas sensor causes the sensor current to fluctuate, and therefore, the leakage current contributes to block the sufficient improvement of measurement accuracy in the conventional gas concentration measuring apparatus.